Known seat frame which can improve a torsional strength and a bending strength is disclosed in a Japanese Patent Laid-Open Publication No. 2001-149175. In the disclosed seat frame, a seat back frame is configured by a pipe forming product and a side bracket is fixed to a side of the seat back frame where a reclining device is provided. The side bracket is formed from steel sheet and a peripheral portion of the side bracket is bent at a right angle so as to form a rib. In addition, a sectional surface of the side bracket has a horseshoe shape. Then, a reinforcing bracket having also a rib and a horseshoe sectional surface is secured between the side bracket and a lower edge of the seat back frame for improving a torsional strength and a bending strength of the side bracket.
According to a seat structure in which a seat belt is mounted to the seat back frame, the seat frame receives an excessive load in case of a vehicle collision. Thus, in the seat frame disclosed in the publication and configured in an aforementioned manner, the strengths of the side bracket and the reinforcing bracket are required to be improved. In order to meet this requirement, however, the sizes of the side bracket and the reinforcing bracket having the horseshoe sectional surface or plate materials thereof need to be larger or thicker.